Conventional techniques for supplying an oxygen gas and a nitrogen gas into an ozone generation apparatus in order to generate an ozone gas are shown in, for example, Patent Documents 1 to 4.
In the techniques shown in Patent Documents 1 to 4, a raw material gas is supplied to an ozone generation apparatus. Here, the raw material gas contains a nitrogen gas of not less than several hundred PPM (and more, several thousand PPM) added to an oxygen gas. In the ozone generation apparatus, an ozone gas is generated from the raw material gas by using a silent discharge. In this manner, in the techniques according to Patent Documents 1 to 4, adding the nitrogen gas of not less than several hundred or several thousand PPM to the oxygen gas allows generation of a high-concentration ozone gas.
As shown in FIG. 12, an ozone generation apparatus 330 according to Patent Documents 1 to 4 includes discharge surfaces (main surfaces that confront electrodes 301a and 301b) facing a discharge space where the silent discharge occurs. At least one side of the discharge surface, a dielectric material 302 made of alumina ceramic or the like is formed (however, the discharge surface facing the discharge space of the ozone generation apparatus 330 includes an ordinary metal material or insulating material having no photocatalytic material). In the discharge space of the ozone generation apparatus 330, an AC voltage is applied to the oxygen gas (raw material gas) containing a large amount of nitrogen as illustrated in Patent Documents 1 to 4, thereby causing a silent discharge. As a result, the ozone generation apparatus 330 generates a high-concentration ozone gas.
In the ozone generation apparatus 330 of Patent Documents 1 to 4, a mechanism of ozone generation is explained based on the theory of particle collisional dissociation. Here, the theory of particle collisional dissociation explains a detailed mechanism for generating a high-concentration ozone gas from an oxygen gas containing a nitrogen gas added thereto. This theory explains a mechanism in which generation of a short-gap silent discharge achieves a high electric field discharge so that an electron having high energy collides with an oxygen molecule, resulting in dissociation of an oxygen gas.
In the ozone generation based on the theory of particle collisional dissociation, a high-concentration ozone gas can be generated irrespective of the amount of added nitrogen contained in a raw material oxygen gas (irrespective of whether or not nitrogen is added). However, the following fact has been confirmed experimentally. That is, in a case where an oxygen gas to which a large amount of nitrogen gas is not added is adopted as a raw material gas, a high-concentration ozone gas cannot be generated even though a short-gap silent discharge is achieved in the ozone generation apparatus 330. For example, in the actual ozone generation apparatus 330, use of an oxygen gas containing no nitrogen results in a failure to generate a high-concentration ozone gas of about 200 g/m3 (160000 ppm) or more, and merely a low-concentration ozone gas of about 20 g/m3 (9333 ppm) can be generated.
Therefore, a mechanism for generating the high-concentration ozone gas in the ozone generation apparatus 330 configured as shown in FIG. 12 cannot be sufficiently explained by the theory of particle collisional dissociation.
The fact that the theory of particle collisional dissociation cannot explain the mechanism of ozone gas generation in the ozone generation apparatus 330 will be detailed little more.
In a silent discharge (dielectric material barrier discharge), generally, a high electric field plasma is obtained depending on a discharge gap length d and gas pressure P. In the ozone generation apparatus 330, generally, the discharge gap d is in a range from several mm to 0.05 mm that is a short gap, and the gas pressure P is in a range from the atmospheric pressure (0.1 MPa) to 0.4 MPa. In the dielectric material barrier discharge under the discharge gap length d and the gas pressure P within such ranges, the plasma density of ion/electron is about 108 (/cm3) to 1010 (/cm3).
In a case where a raw material gas containing a large amount of nitrogen (about 10000 ppm) added to an oxygen gas is supplied into plasma with a plasma density of 1010 (/cm3), an ozone molecule density σmax (/cm3) that provides a maximum ozone concentration of 290 g/m3 (135000 ppm) shown in FIG. 13 which will be described later is as follows, based on the theory of particle collisional dissociation (the oxygen gas and electrons repeatedly and frequently collide with each other, to dissociate the oxygen gas into oxygen atoms).
The ozone molecule density σmax={1.35×105/106}×6.02×1023/2.24×104=˜3.63×1018 (/cm3).
In a case where a raw material gas containing a small amount of nitrogen (about 1 ppm) added to an oxygen gas is supplied into plasma with a plasma density of 1010 (/cm3), an ozone molecule density min (/cm3) that provides an ozone concentration of 40 g/m3 (32000 ppm) shown in FIG. 13 which will be described later is as follows, based on the theory of particle collisional dissociation (the oxygen gas and electrons repeatedly and frequently collide with each other, to dissociate the oxygen gas into oxygen atoms).
The ozone molecule density σmin={3.2×104/106}×6.02×1023/2.24×104=8.6×1016 (/cm3).
Thus, under the same plasma density, addition of a large amount of nitrogen (10000 ppm) generates ozone having a molecule density of 3.63×1018 (/cm3), while adding a small amount of nitrogen (1 ppm) generates ozone having a molecule density of 8.6×1016 (/cm3).
As described above, the molecule density of generated ozone varies in double digits depending on the amount of added nitrogen. This result means that the mechanism of ozone generation in the ozone generation apparatus 330 is not sufficiently explained by the theory of particle collisional dissociation that is based on a high electric field discharge.
In this respect, Patent Document 5 discloses that the mechanism of ozone gas generation in the ozone generation apparatus 330 is based on a catalytic activity.
As described above, under the condition of the plasma with a plasma density of 1010 (/cm3), it is assumed that the ozone molecule density is 1014 to 1016 (/cm3) order when the theory of particle collisional dissociation is adopted. However, the actual ozone molecule density is as high as 1018 (/cm3). Therefore, it is inferred that, due to the behavior of a molecule of the nitrogen gas itself, an effect of a chemical action (for example, a catalytic reaction of nitrogen) contributes so that the above-mentioned high concentration ozone gas is generated. Based on this inference, Patent Document 5 explains a mechanism of generation of a high concentration ozone gas (addition of a nitrogen gas causes generation of a large amount of oxygen atoms because of a photochemical reaction between a small amount of an NO2 gas and an NO gas that are generated during a discharge, resulting in generation of a high-concentration ozone gas). In the following, a slight mention will be made of the disclosure of Patent Document 5.
Patent Document 5 discloses such a mechanism that an ozone gas is generated due to a catalytic activity like a chemical reaction caused between a large amount of nitrogen gas itself contained in an oxygen gas and a discharge, based on the experimental fact that adding a large amount of nitrogen gas (for example, 1% (10000 ppm)) provides a high-concentration ozone gas of about 200 g/m3 (160000 ppm) or more and the experimental fact that a high concentration ozone gas of 16 times or more is generated in accordance with the amount of an added nitrogen gas.
Patent Document 5 describes a chemical reaction in which an NO2 gas is generated from a nitrogen gas as a result of the discharge, as follows.
That is, when an oxygen gas containing a nitrogen gas is supplied into a discharge space where a silent discharge is occurring, an NO2 gas is generated based on the following Reaction Formulae 1 and 2. N2+e→2N+ (the reaction of ionizing a nitrogen molecule; Reaction Formula 1). 2N++O2+M→>NO2 (the reaction of generating NO2; Reaction Formula 2). Based on these Reaction Formulae 1 and 2, an NO2 gas of several ppm to several tens ppm is generated.
Moreover, Patent Document 5 discloses that: the NO2 gas generated as a result of the reactions indicated by Reaction Formulae 1 and 2 causes a photodissociation reaction (a catalytic activity like a photochemical reaction) due to discharge light wavelength energy (hv), resulting in generation of an NO gas and an oxygen atom (O) (Reaction Formula 3); the generated NO gas causes an oxidation reaction with an oxygen molecule, resulting in generation of an oxygen atom (O) and an NO2 gas (Reaction Formula 4); and Reaction Formula 3 and Reaction Formula 4 are alternately repeated, to thereby generate a large amount of oxygen atoms (O).
Here, Reaction Formula 3 is NO2+hv→NO+O (photodissociation reaction of NO2), and Reaction Formula 4 is NO+O2 (an oxygen gas that is a primary component of the raw material gas) NO2+O (an oxidation reaction of NO).
In the technique according to Patent Document 5, when the amount of added nitrogen is 4% or more of the oxygen gas, the NO2 gas of 400 ppm or more is generated as a result of the discharge, and this NO2 gas reacts with oxygen (and generated ozone), so that a large amount of a nitrogen compound gas (NOx gas) such as an N2O5 gas is generated. As a result, as shown in the following Reaction Formula 5 rather than Reaction Formulae 3 and 4 that are the dissociation reaction into oxygen atoms, a chemical reaction between the generated ozone and NOx gas accounts for an increased percentage, which causes an ozonolysis reaction at an accelerated rate. Reaction Formula 5 is O3+N2O5→2O2+2NO2 (decomposition of ozone due to impurities).
In this manner, in the technique according to Patent Document 5, when the amount of the nitrogen gas added to the raw material gas exceeds 4%, a large amount of NOx gas such as an NO2 gas, which is a by-product caused by the discharge, is generated. This increases the percentage of the chemical reaction between the generated ozone gas and NOx gas, which promotes the action for decomposing the generated ozone gas, resulting in a failure to extract a high-concentration ozone gas.
As described above, Patent Document 5 discloses, instead of the ozone gas generation mechanism based on the theory of particle collisional dissociation, the ozone gas generation mechanism based on the catalytic activity like a chemical reaction (a large amount of dissociation into oxygen atoms is caused due to a photochemical reaction using the generated nitrogen compound gas and the discharge light, and the oxygen atoms obtained as a result of the dissociation are efficiently bound with oxygen molecules, so that a high-concentration ozone gas is generated).
FIG. 13 shows the relationship between the rate γ of nitrogen addition in the oxygen gas and the concentration of the ozone gas generated in the ozone generation apparatus 330. A result shown in FIG. 13 was obtained under conditions that, in the ozone generation apparatus 330 shown in FIG. 12, the short-gap discharge space was 0.1 mm and the gas pressure was 0.25 MPa, and a silent discharge was caused.
FIG. 13 reveals that, in the ozone generation apparatus 330 shown in FIG. 12, a decrease in the rate γ of nitrogen addition accordingly decreases the concentration of the generated ozone gas.
Accordingly, in order to suppress a decrease in the concentration of the generated ozone gas even when the amount of added nitrogen gas is reduced in the ozone generation apparatus 330, it is necessary to change the gap of the discharge space, the gas pressure, the area of the discharge surface, and the like (for example, an increase in the discharge surface area enables generation of an ozone gas having a higher concentration).
Here, even when the gap of the discharge space is changed in a range from 0.05 mm to several mm, the gas pressure is also changed, and/or the area of the discharge surface is also changed, the graph shown in FIG. 13 still has the same shape. Such changes merely cause the solid line graph shown in FIG. 13 to move up and down.
For example, in FIG. 13, when the rate of nitrogen addition is 10000 ppm, the concentration of the generated ozone gas is 290 g/m3 (135000 ppm), and when the rate of nitrogen addition is 1 ppm, the concentration of the generated ozone gas is 40 g/m3 (32000 PPM). Even if the gap of the discharge space, the gas pressure, the area of the discharge surface, and the like, are changed in the above-described manner, the concentration of the ozone gas generated when the rate of nitrogen addition is 10000 ppm and the concentration of the ozone gas generated when the rate of nitrogen addition is 1 ppm are increased and decreased, respectively, but, for example, the ratio of “(the concentration of the ozone gas generated when the rate of nitrogen addition is 10000 ppm)/(the concentration of the ozone gas generated when the rate of nitrogen addition is 1 ppm)” is unchanged.
Patent Document 6 shows an ozone generation apparatus that adopts an ozone gas generation method different from that of the ozone generation apparatus 330 shown in FIG. 12. Patent Document 6 discloses an ozone generation apparatus (called a nitrogen-free ozone generation apparatus) that generates a high-concentration ozone gas by using a raw material gas that contains only a high purity oxygen gas.
In the ozone generation apparatus according to Patent Document 6, a photocatalytic material is applied to a discharge surface facing a discharge space where a silent discharge occurs. When an AC voltage is applied to the oxygen gas supplied to the discharge space of the ozone generation apparatus and a silent discharge is caused, light having a wavelength of the visible light region (visible light of 428 nm to 620 nm) is emitted (discharged). The photocatalytic material absorbs the light having a wavelength of the visible light region which is emitted in the discharge. As a result, the discharge surface having the photocatalytic material, over which the gas passes, is excited, to exert a photocatalytic activity function. A photocatalytic effect of the photocatalytic material dissociates the oxygen gas in the discharge space. A chemical reaction between oxygen atoms obtained as a result of the dissociation and oxygen molecules contained in the oxygen gas generates a high-concentration ozone gas. Details of a mechanism and a configuration for generating a high-concentration ozone gas from a high-purity oxygen gas by using a photocatalytic material are known, as disclosed in Patent Document 6.
For example, in the field of semiconductor fabrication, the ozone gas generated in each of the above-described ozone generation apparatuses is used for an ozone treatment process such as formation of an ozone oxide insulating film and ozone washing, which is performed in a treatment apparatus provided separately from the ozone generation apparatus.
In the two different ozone gas generation apparatuses (the ozone generation apparatus 330 shown in FIG. 12 and the nitrogen-free ozone generation apparatus) described above, a voltage applied across electrodes that form the discharge space was applied in a stepwise manner, and a rise in the ozone concentration of the generated ozone in such a case was examined. A result thereof is shown in FIG. 14.
In FIG. 14, the broken line indicates ozone concentration rise characteristics obtained when a raw material gas containing a large amount of nitrogen gas added was supplied to the ozone generation apparatus shown in FIG. 12. In FIG. 14, the solid line indicates ozone concentration rise characteristics obtained when a raw material gas containing only a high-purity oxygen gas was supplied to the nitrogen-free ozone generation apparatus shown in FIG. 11 which will be described later.
As indicated by the solid line in FIG. 14, in the nitrogen-free ozone generation apparatus, when the voltage is applied stepwise, the ozone concentration promptly rises, and within about five seconds, reaches a steady-state concentration. On the other hand, in the ozone generation apparatus 330 shown in FIG. 12, when the voltage is applied stepwise, the ozone concentration rises after a delay of several seconds, gradually and asymptotically approaches a steady-state value, and about two to three minutes later, reaches a steady-state concentration.
Thus, the two ozone gas generation apparatuses described above exhibit different ozone concentration rise characteristics, and the difference in the ozone concentration rise characteristics is due to a difference in the mechanism for generating the ozone gas.
In the ozone generation apparatus 330 shown in FIG. 12, as seen from the ozone gas generation mechanism based on the Reaction Formula 1 to 4 mentioned above, generation of the NO2 gas from the raw material gas containing a nitrogen gas added to an oxygen gas is once caused by the discharge, and then the ozone gas is generated by the catalytic activity. In this ozone gas generation mechanism, the rise in the ozone concentration is delayed when the stepwise voltage is applied as indicated by the broken line in FIG. 14.
On the other hand, in the nitrogen-free ozone generation apparatus, when the stepwise voltage is applied and the silent discharge is caused, the discharge light is immediately emitted. This discharge light promptly excites the photocatalytic material (reference numeral 303 in FIG. 11). This enables the photocatalytic effect to be immediately exerted, which can dissociate the supplied raw material gas (oxygen gas) into oxygen atoms due to the photocatalytic effect, so that the ozone gas is generated. This ozone gas generation mechanism of the nitrogen-free ozone generation apparatus enables the ozone concentration to promptly rise when the stepwise voltage is applied.
As thus far described, each of the ozone generate mechanisms has been demonstrated in the experiment shown in FIG. 14.